PDCD1 and cancer: In recent years, the tumor microenvironment (TME) and relevant immune checkpoint inhibitors (ICIs), which target programmed cell death protein 1 (PD-1) and its ligands (PD-L1) and the cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), have gradually become a focus of research in cancer treatment (7, 8).